The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are a substrate 10, which could be a lower magnetic shield and/or a lower lead, on which is seed layer 11. Antiferromagnetic (AFM) layer 12 is on seed layer 11. Its purpose is to act as a pinning agent for a magnetically pinned layer. The latter is typically a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a non-magnetic spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. One of the important parameters that defines a CPP device is R.A, the resistance area product. This can be improved by means of a spacer layer that comprises an NOL (nano-oxide layer) 162 sandwiched between two layers of copper 161 and 163, as shown in FIG. 2. Since conduction between the two copper layer occurs only where there are pin-holes in the NOL, a higher transverse resistance is obtained without any reduction in overall device area.
Capping layer 18 lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase.
Earlier GMR devices were designed to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have also emerged. CPP GMR heads are considered to be promising candidates for the over 100 Gb/in2 recording density domain (see references 1-3 below).
It is known [3] that Fe rich CoFe such as CoFe(50%), when used in AP1 or the free layer, can enhance CPP GMR due to high spin polarization. However, it has also been found that using CoFe(50%) for AP1 resulted in poor EM (electro-migration) and larger device to device variation. We believe this happens because CoFe(50%) has a preferred bcc crystalline orientation whereas typical spin valve seed layers prefer an fcc type growth orientation.
The present invention discloses a way to overcome this problem
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,714,388, Hasegawa et al. disclose that a CoFe layer having a Fe per-centage of greater than 15 degrades magnetic performance. U.S. Pat. No. 6,710,984 (Yuasa et al.) discloses a CoFe. layer having a Fe percentage of 10. Lubitz et al. disclose a CoFe layer having a Fe percentage of 5 in U.S. Pat. No. 6,171,693. No references that teach an Fe percentage of greater than 50 were found.
References
    [1] M. Lederman et al. U.S. Pat. No. 5,627,704.    [2] J. W. Dykes et al. U.S. Pat. No. 5,668,688    [3] H. Yuasa et al. J. A. P. 92 2002 p. 2646